IFMIB(4) FreeBSD Kernel Interfaces Manual IFMIB(4)


ifmibManagement Information Base for network interfaces


#include < sys/types.h>
#include < sys/socket.h>
#include < sys/sysctl.h>
#include < sys/time.h>
#include < net/if.h>
#include < net/if_mib.h>


The ifmib facility is an application of the sysctl(3) interface to provide management information about network interfaces to client applications such as netstat(1), slstat(8), and SNMP management agents. This information is structured as a table, where each row in the table represents a logical network interface (either a hardware device or a software pseudo-device like lo(4)). There are two columns in the table, each containing a single structure: one column contains generic information relevant to all interfaces, and the other contains information specific to the particular class of interface. (Generally the latter will implement the SNMP MIB defined for that particular interface class, if one exists and can be implemented in the kernel.)

The ifmib facility is accessed via the “ net.link.generic” branch of the sysctl(3) MIB. The manifest constants for each level in the sysctl(3) name are defined in < net/if_mib.h>. The index of the last row in the table is given by “ net.link.generic.system.ifcount” (or, using the manifest constants, CTL_NET, PF_LINK, NETLINK_GENERIC, IFMIB_SYSTEM, IFMIB_IFCOUNT). A management application searching for a particular interface should start with row 1 and continue through the table row-by-row until the desired interface is found, or the interface count is reached. Note that the table may be sparse, i.e., a given row may not exist, indicated by an errno of ENOENT. Such an error should be ignored, and the next row should be checked.

The generic interface information, common to all interfaces, can be accessed via the following procedure:

get_ifmib_general(int row, struct ifmibdata *ifmd) 
 int name[6]; 
 size_t len; 
 name[0] = CTL_NET; 
 name[1] = PF_LINK; 
 name[2] = NETLINK_GENERIC; 
 name[3] = IFMIB_IFDATA; 
 name[4] = row; 
 name[5] = IFDATA_GENERAL; 
 len = sizeof(*ifmd); 
 return sysctl(name, 6, ifmd, &len, (void *)0, 0); 

The fields in struct ifmibdata are as follows:

( char []) the name of the interface, including the unit number
( int) the number of promiscuous listeners
( int) the interface's flags (defined in < net/if.h>)
( int) the current instantaneous length of the send queue
( int) the number of packets dropped at this interface because the send queue was full
( struct if_data) more information from a structure defined in < net/if.h> (see if_data(9))

Class-specific information can be retrieved by examining the IFDATA_LINKSPECIFIC column instead. Note that the form and length of the structure will depend on the class of interface. For IFT_ETHER, IFT_ISO88023, and IFT_STARLAN interfaces, the structure is called “ struct ifmib_iso_8802_3” (defined in < net/if_mib.h>), and implements a superset of the RFC 1650 MIB for Ethernet-like networks.


sysctl(3), intro(4), ifnet(9)

F. Kastenholz, Definitions of Managed Objects for the Ethernet-like Interface Types Using SMIv2, August 1994, RFC 1650.


The ifmib interface first appeared in FreeBSD 2.2.


Many Ethernet-like interfaces do not yet support the Ethernet MIB; the interfaces known to support it include ed(4) and de(4). Regardless, all interfaces automatically support the generic MIB.
October 28, 2009 FreeBSD